This captures the sales referrer and stores it along with the order. The referrer can then be seen on the at the bottom of the order > Order Details tab.

Referrer is captured with the use of a cookie.

This comes with a vQmod but you will also have to edit the index.php file and add a field to the order table via your phpMyAdmin. There are some basic instructions to do this.

Along with capturing the referrer you can also specify a referrer by adding ?referrer=whateveryouwanthere to the end of the url.

I have given this a quick test on 1.5.4.1 and it seems to be working OK. Should work on most other 1.5.x versions